Low temperature refrigeration in ethylene plants



July 6, 1965 P. CAHN LOW TEMPERATURE REFRIGERATION IN ETHYLENE PLANTS Filed April 24, 1961 EXPANSION VALVE ETHYLENE DISTILLATION TOWER HYDROGEN, 32 METHANE /49 PROPYLENE 29 R H -l8 7-- ABSORPTION /47 TOWER 7 34 a 1 ,IDEETHANIZER u PROPYLENE;/|4

Robert P. Cohn ETHANE INVENTOR PATENT ATTORNEY United States Patent C) 3,192,732 LQW TEMPERATURE REFRIGERATESN IN ETHYLENE PLANTS Robert P. Calm, Millbnrn, N.J., assignor to Essa Research and Engineering Company, a corporation of Delaware Filed Apr. 24, 1961, Ser. No. 105,144 11 Claims. (Cl. 6217) The present invention relates to an improved method for utilizing at low temperatures the refrigeration available in a high pressure liquid ethane stream. More particularly, this invention relates to cooling the said liquid ethane stream to a temperature below F., followed by expanding it into a large volume of a colder light gaseous stream to thereby obtain vaporization of the liquid stream and further cooling of the combined stream. Most particularly, in a preferred embodiment, this invention relates to an improved light ends system for recovering propylene and ethylene from a C stream from steam cracking. In this system comprising a first demethanization by absorption followed by successive fractionations the liquid ethane stream from the ethylene-ethane splitter is cooled to a temperature below -20 F. and is flashed in the presence of the expanded cold 110 to 140" F. tail gas from the absorber demethanizer to provide additional low temperature refrigeration for the process.

According to the present invention it has now been discovered that large savings in requirements for low temperature refrigeration, i.e. refrigeration at 100 to 150" R, may be obtained by cooling the liquid ethane product to a temperature below 0 F., preferably below 20 and flashing this ethane in the presence of a large amount of a lighter gas. Preferably, this gas is the cold tail gas from a demethanizer containing mainly methane and hydrogen. This gas is conventionally combined with the ethane and burned as a fuel gas so that no disadvantage is incurred in combining the two streams. It should be noted that at these low temperatures in the absence of the additional light gases expansion to 5010O p.s.i.g. (fuel gas pressure) would not produce vaporization of ethane since the vapor pressure of ethane at these temperatures is lower than these pressures.

Ethane vapor pressures at various temperatures are listed below:

By the present expedient the effective pressures are reduced in accordance with Daltons Law as follows: partial pressure of ethane total pressure mol fraction of ethane. It can be seen that for practical expansion, vaporization of liquid ethane to pressures of 50-150, preferably 50-100 p.s.i.g., the relative proportion of the light gases should be 1:1 to 25: 1, preferably 2:1 to 20:1, e.g. 6:1 mols of light gas per mol of liquid ethane. At these dilutions, ethane can be evaporated at 50-100 p.s.i.g. at temperatures of 100 F. to -l50 PI, thus making refrigeration available at these low temperatures equal to the latent heat of the ethane evaporated. Without the diluting gas, ethane would evaporate at only 68 F. to 38" F., corresponding to 50 and 100 p.s.i.g., seriously degenerating the available refrigeration to a less attractive level.

A particularly advantageous arrangement is to precool the liquid ethane to as low a temperature as possible by heat exchange prior to mixing with the cold light gas stream. Thus, if the ethane is to be flashed into a 100 to 150 F. gas stream, it is most advantageous to preoool the ethane as near as possible to the 100 F. to 150 F. range. This will make the maximum use of "ice Refrigeration Available at 120 F. or Lower Due to lb. Moles of Liquid Ethane Temperature of Ethane Before Flashing into F. Ta Gas The amount of heat which has to be abstracted from the liquid ethane prior to flashing is equal to that gained by the refrigeration capacity below 120" F. However, this precooling is at a much higher temperature level than the 120" F. refrigeration increase, so that a net gain is achieved resulting in appreciable power savings.

The principal and preferred use of this system, is in the separation of valuable ethylene from a gas containing ethylene in combination with methane and hydrogen. Thus, effluent gases from hydrocarbon steam cracking processes, e.g. C streams can be more efficiently separated according to the present invention. Specifically, the present invention aids in the recovery of propylene and other light hydrocarbons from the hydrogen-methane tail gas without the need for expensive equipment such as turbo-expanders or ethylene refrigeration.

It is known that in the separation of ethylene from gaseous mixtures containing hydrogen, methane, ethane and/or ethylene, propane and/or propylene in combination with higher boiling hydrocarbons, the principal difficulty of separation lies in the elimination of methane and hydrogen from said mixture. In the prior art it has been found advantageous to concentrate the C components, which are mainly propylene, e.g. 95 mol percent propylene, 5 mol percent propane, and to utilize this stream as a solvent for the C components, such as ethane and ethylene. In this manner the separation between methane and the C hydrocarbons is facilitated. However, in order to accomplish such separation, low temperature refrigerant is required in the distillation-absorption process. Thus, the temperature at the top of the demethanizer, in such a process will ordinarily be of the order of 112 F. Such low temperatures are required in order to avoid an excessive loss of C hydrocarbons in the efiluent gas and to attain these temperatures, ethane or ethylene refrigeration rather than only propylene refrigeration is required. Alternatively, according to the prior art the temperature at the top of the demethanizer may be maintained at about 0 to 20 F., but at these temperatures large quantities of propylene are taken overhead. In the latter prior art system to reduce this propylene loss with the overhead gas, the overhead stream must be processed in either of two ways: 1) the gas may be separately cooled with ethane or ethylene refrigeration to condense the propylene, or (2) the gas may be cooled by refrigeration provided by sending the overhead gas through turbo expansion engines and using merely recycled propylene refrigeration.

In the present process additional refrigeration at the necessary low temperatures is made available from the liquid ethane from the ethylene splitter thus reducing the requirement for either (1) extraneous ethane or ethylene refrigeration, or (2) turbo expander refrigeration.

The present invention will be more clearly understood from a consideration of the accompanying drawing wherein a preferred process for carrying out the invention is diagrammatically illustrated. A feed stream of C and ligher components obtained for example from steam cracking is supplied at a temperature of 35 F. to F., e.g. 25 F. through line l to the middle part of the absorber 2 operated at 300 to 600 p.s.i.a., specifically 420 p.s.i.a. This feed stream may contain, for example, 224 mols/hr. H 365 mole/hr. CH 428 mols/hr. ethylene, 104 mols/hr. ethane, 282 mols/hr. propylene, 21 mcls/hr. propane, 0.5 mols/hr. CO. A propylene lean oil stream supplied from a deethanizer as will be described is introduced at a temperature of to 35 F., e.g. F. through line 3 to the overhead stream from the absorber. The amount of this lean oil stream may be for the above example, 650 lb. mols/ hr. From the bottom of the tower the demethanized C stream is removed through line 4, a part of said stream being supplied through line 5, reboiler 6 and line 7 back to the tower and the remainder being passed through line 8 at a temperature of 60 to 90 F., e.g. 80 F. to deethanizer 9. This deethanizer is operated at a pressure of 300 to 600 p.s.i.g., e.g. 370 p.s.i.g. From the bottom of the deethanizer a propylene stream (containing also the small amounts of propane present in the C cut) is passed through line 10, part of the stream being passed back to the column through line 11, reboiler 12 and line 13, and of the remainder, part being passed back to column 2 and the remainder being taken oil as product through line 14. The liquid propylene passed back to absorption tower 2 is passed through line 15 to cooler 16 where its temperature is reduced to 0 to 20 F., e.g. 5 F. From the cooler the liquid propylene is passed through line 17 to join the vapors taken overhead from tower 2 through line 18. The combined stream is passed through line 19 to condenser 20 where the temperatureis reduced to l5 to 35 F., e.g. 20 F. The cooled stream is passed through line 21 to separator 22 and the separated liquid is recycled to the column through line 3. The overhead gases from separator 22, are passed through line 23 to cooler 24, where the temperature of the gas is reduced to 80 to 120 F., e.g. 105 F. From cooler 24 the gases are supplied through line 25 to Joule-Thompson expansion valve 26 where the pressure is reduced to 50 to 150 p.s.i.g., e.g. 100 p.s.i.g. Reduced pressure and liquid gas at a temperature of l00 to 150 F., eg. 130 F. are supplied through line 27 to Joule-Thompson recovery drum 28 where liquid propylene is recovered through line 29. The overhead gases are passed through line 30 where they are further cooled to a temperature of 110 to 160 F., e.g. 135 F. by the vaporization of a liquid ethane stream supplied through line 31 as will be described. The combined gaseous or gas-liquid stream is supplied through line 32 back to cooler 24 where these low temperature gases or gas-liquid mixture are utilized to provide refrigeration. The gases leave cooler 24 through line 33. These heat exchange gases in line 33 are now at a temperature of 25 to F., e.g. -30 F. Alternatively to a single cooler a number of staged coolers may be used as is conventional in the art.

Returning now to the deethanizer 9 the C stream is passed through line 34 to condenser 35, a part of the stream being refluxed back to the column through line 36 and the remainder being passed through line 37 to ethylene separation column 38. This column may operate in the range of 75 to 400 p.s.i.g. From this column the ethylene stream is passed overhead through line 39 to condenser 40, a suitable reflux stream being returned through line 4-1 and the remainder being passed to product through line 42. From the bottom of the column a part of the ethane stream after return of a reboil stream through line 43,-reboiler 44 and line 45 is passed through line 45 as a product to fuel gas or to various uses such as feed material for steam cracking and the remainder of the stream at temperature of 20 to 30 F., e.g. 25 F. is passed through line 47, cooled in exchanger 48 to 0 F. to 150 F., e.g. 23 F. and is passed through line 49 to expansion valve 50. Here the liquid is vaporized and expanded in line 31 to a pressure of 50 to 150 p.s.i.g., e.g. p.s.i.g., in the presence of overhead vapors, from the Joule-Thompson recovery drum 2 8, supplied through line 30 as previously described.

It should be noted that by the present invention process it is possible to either (1) cool the vapors to lower temperatures thus recovering more propylene in Joule-Thompson recovery drum 28 or (2) reduce the requirement for extraneous low temperature refrigeration required in the refinery. Thus, alternatively to utilizing all the low temperature refrigeration in cooler 24 it may be utilized in other cooling operations in the refinery.

An example of the improvement obtained by the present process can be seen from a comparison of a prior art processing scheme with that of the present invention. In a prior art scheme refrigeration is recovered out of the spent ethane in an ethylene recovery plant by flashing the ethylene/ethane splitter bottoms down to fuel gas pressure (50-75 p.s.i.g.), exchanging the latent heat (at 50 F.), and then heating the gas by exchange up to 10 to 30 F. before mixing with the tail gas. In the present scheme the liquid ethane and the cold (l10 or l40- F. or lower) absorber-demethanizer tail gas are mixed at fuel gas pressure, and then the mixture is exchanged. Thus, all the latent heat of the ethane product is available as refrigeration at the lowest ethylenerefrigeration level (say 140 F. or so), since ethane will evaporate into the tail gas at that temperature (its partial pressure is 10-15 psi).

The refrigeration recovered in the two processing schemes is compared below.

Recover able refrigeration, mm. ELLA/1'17).

*Part of the refrigeration available from the combined tail gas +01 stream is used to cool the splitter bottoms from 33 F. to -150 F. prior to mixing with the cold tail gas.

A particularly attractive alternate is to use the liquid propylene from the .J oule-Thompson recovery drum 28, i.e. line 29, to cool the liquid ethane prior to flashing through valve 50. In the present example, the stream in line 29 is at F. and the ethane fed to valve 50 at 23 F. By suitable heat exchange between these two streams, the temperature of the liquid ethane fed to valve 50 can be lowered to l00 F. to 1l0 F. For the quantities indicated lb. mols/hr. of ethane), this will make an additional 230,000 B.t.u./hr. of low temperature refrigeration available in the 100 F. to 130 F. temperature range.

The foregoing description contains a limited number of embodiments of the present invention. It will be understood that this invention is not limited thereto since numerous variations are possible without departing from the scope of the following claims.

What is claimed is:

1. An improved process for utilizing at low temperatures the refrigeration available in a high pressure liquid ethane stream which comprises flashing the liquid ethane stream at a temperature in the range of 0 to F. and from a pressure in the range of 75 to 400 p.s.i.g. to a pressure in the range of 50 to 150 p.s.i.g. in the presence of a cool lighter gaseous stream supplied at a temperature below 80 F. and utilizing the combined stream to effect refrigeration of another stream.

2. The process of claim 1 in which the relative proportion of the lighter gaseous stream is in the range of 1:1 to 20:1 mols of the lighter stream per mol of the liquid ethane stream.

3. The process of claim 1 in which the lighter gaseous stream comprises essentially hydrogen and methane.

4. The process of claim 1 in which the initial pressure of the liquid ethane is in the range of 150 to 350 p.s.i.g.

5. An improved demethanization process which comprises passing a C3- stream containing hydrogen and methane to an absorption tower operated at a pressure of 300 .to 600 p.s.i.g. supplying a liquid propylene lean oil to the top of said tower to absorb materials heavier than methane from said C stream, expanding the overhead stream from said tower to a pressure in the range of 50 to 150 p.s.i.g. to effect cooling of said stream, further cooling said expanded stream to a temperature below 80 F., and expanding a liquid ethane stream at a temperature of to 150 F. in the presence of the cooled expanded overhead vapor stream from said tower from a pressure in the range of 75 to 400 p.s.i.g. to a pressure in the range of 50 to 150 p.s.i.g. to effect vaporization of the liquid ethane stream and further cooling of the combined stream and utilizing the combined stream to effect refrigeration of another stream.

6. The process of claim 5 in which the relative proportion of the cooled overhead stream from the tower to the liquid ethane stream is in the range of 1:1 to 20:1 mols of the former per mol of the latter.

7. The process of claim 5 in which the C stream is obtained by fractionation from a product stream from steam cracking.

8. The process of claim 5 in which the liquid ethane and the liquid propylene lean oil are separately obtained from the liquid propylene oil containing absorbed materials heavier than methane by fractionation.

9. The process of claim 5 in which liquid propylene is separated by distillation from the liquid propylene oil containing absorbed materials heavier than methane, a part of the liquid propylene separated is passed back to the absorption tower as the propylene lean oil supplied to said tower, liquid ethane is separated by distillation from the overhead vapors from said distillation separation of liquid propylene, and at least a major part of the liquid ethane separated is passed to the liquid ethane expansion.

10. The process of claim 5 in which the expanded overhead stream from the absorption tower is further cooled to 100 F. to l F. prior to being passed to the liquid ethane expansion vaporization.

11. The process of claim 5 in which the liquid ethane is precooled before flashing to a temperature in the range of to F.

References Cited by the Examiner UNITED STATES PATENTS 1,325,667 12/19 Crawford 62--114 2,028,432 1/36 Barton 6217 X 2,573,341 10/51 Kniel 62--17 X 2,731,810 1/56 Hachmuth 62-31 X 2,777,305 1/57 Davidson 6217 2,804,488 8/57 Cobb 62-17 X 2,813,920 11/57 Cobb 55-51 2,915,881 12/59 Irvine 6217 NORMAN YUDKOFF, Primary Examiner.

ROBERT A. OLEARY, Examiner. 

1. AN IMPROVED PROCESS FOR UTILIZING AT LOW TEMPERATURES THE REFRIGERATION AVAILABLE IN A HIGH PRESSURE LIQUID ETHANE STREAM WHICH COMPRISES FLASHING THE LIQUID ETHANE STREAM AT A TEMPERATURE IN THE RANGE OF 0 TO -150* F. AND FROM A PRESSURE IN THE RANGE OF 75 TO 400 P.S.I.G. TO A PRESSURE IN THE RANGE OF 50 TO 150 P.S.I.G. IN THE PRESENCE OF A COOL LIGHTER GASEOUS STREAM SUPPLIED AT A TEMPERATURE BELOW - 80*F. AND UTILIZING THE COMBINED STREAM TO EFFECT REFRIGERATION OF ANOTHER STREAM. 